1. Field of the Invention
The present invention relates to a ladder filter including plurality of elastic wave resonators and a duplexer that includes the ladder filter as a transmission filter.
2. Description of the Related Art
Ladder surface acoustic wave filters have been used for, for example, transmission filters included in duplexers in cellular phones.
Japanese Unexamined Patent Application Publication No. 2000-196412 and Japanese Unexamined Patent Application Publication No. 8-307191 disclose ladder filters including plurality of resonators composed of interdigital transducer (IDT) electrodes. In Japanese Unexamined Patent Application Publication No. 2000-196412, a line including a busbar of one IDT electrode and a line including a busbar of another IDT electrode are arranged so as to not be overlapped with each other when viewed from a plan view. In contrast, in Japanese Unexamined Patent Application Publication No. 8-307191, a busbar of one IDT electrode and a busbar of another IDT electrode are arranged so as to be overlapped with each other with a dielectric thin film interposed therebetween when viewed from a plan view.
However, since the lines are arranged so as to not be overlapped with each other in the ladder filter in Japanese Unexamined Patent Application Publication No. 2000-196412, it is difficult to reduce the size of the ladder filter.
In contrast, since the busbars having thin film thicknesses are overlapped with each other with a dielectric thin film interposed therebetween, as in electrode finger portions, in Japanese Unexamined Patent Application Publication No. 8-307191, the resistance component can be increased in size. Accordingly, the loss can be increased.